WSP12655

• File Number: 8220.101.10.B
• Description: Colorado River - Water Projects - Glen Canyon Dam-Lake Powell - Glen Canyon TWG
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/2004
• Author: Schmidt - Topping - Grams - Hazel
• Title: System-Wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor Between Glen Canyon Dam and Bright Angel Creek - Arizona - Final Report - 10-01-04
• Water Supply Pro - Doc Type: Report/Study

0.02358

3

2

Elevation of water surface
at discharge shown in
cubic meters per second

I 2830
I
I
!
f---- 1410

i
r--H 850
;

Figure 14. Diagram depicting generalized intemal stratigraphy of Grand Canyon reattachment bars as they existed in
the late 1980s. 1 is the pre-dam deposit whose upper surface was eroded in 1983. 2 is the flood sand of 1983 and
includes fluvial dunes and climbing ripples. 3 is the high flow sand deposited between 1984 and 1986. This deposit
was of limited extent and was generally thin and are primarily climbing ripples. 4 are deposits formed by fluctuating
flows that occurred after spring 1986. From Rubin et al. (1994b, fig. 3).

and by the Controlled Flood of 1996. The size
and characteristics of post-dam flood deposits
have changed with time. The 1965 flood
deposits may have been reworked in 1980 and
were reworked in 1983. Smaller floods of
1984 to 1986 and 1996 reworked the lower
parts of the 1983 deposits.
Stratigraphic relations among these flood
deposits and the lower elevation fluctuating
flow deposits were described by Rubin et al.
(1990, 1994b), Schmidt and Graf (1990), and
Schmidt and Rubin (1995). These field studies
demonstrated that the thickest post-dam flood
deposit, occurring beneath a high topographic
bench, formed in 1983 and was comprised of
well-sorted to very well sorted very-fine to
medium sand (Fig. 14). Although the smaller
floods of 1984 to 1986 created a distinct bench
approximately 1 m lower than the 1983 bench,
the underlying deposit ofvery-fme to medium
sand was typically thin. In upper Grand
Canyon, floods from the Little Colorado River
in winter 1993 (Wiele et aI., 1996) formed
flood deposits 10s of centimeters to several
meters thick. The 1996 Controlled Flood
formed new flood deposits of fine to coarse
sand that were up to several meters thick near
the reattachment points of some eddies. These
deposits occurred at approximately the same

elevation as those of the period between 1984
and 1986, and it was not possible to distin-
guish these deposits in mapping conducted
after 1996. Rubin et aI. (1998) and Topping et
aI. (1999) found that the 1996 Controlled
Flood deposits were inversely graded, consis-
tent with the temporal coarsening of the sus-
pended load of the same flood. Topping et al.
(2000a) identified similar patterns in pre-dam
flood deposits and in those formed by the
experimental release of November 1997.
Fluctuating flow deposits are active bars of
very fine to fine sand inset into the flood
deposits.

4.0 PREVIOUS STUDIES OF FINE-
SEDIMENT FLUX AND STORAGE

Changes in the bed and in the size and
distribution of fine-sediment deposits along the
channel caused by completion of Glen Canyon
Dam are ultimately caused by changes in the
balance between influx and efflux of fine
sediment to and from the study area. Although
changes in the flux have been dramatic, the
resulting changes to the fme-sediment deposits
have been less obvious. Previous studies
reached conflicting conclusions about the long-
term trends of these changes.

20 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ...